Are you sure? It comes as complete news to me. I have followed Tennis for years and if you are correct as I expect you are, it is something I have missed .
1.08 Reduction of ATP World Tour Masters 1000 Commitment
A. A player’s number of ATP World Tour Masters 1000 commitment tournaments shall be reduced by one (1) tournament for reaching each of the following milestones:
1) 600 matches* (as of 1 January of the commitment year)
2) 12 years of service
3) 31 years of age (as of 1 January of the commitment year)
If all three (3) conditions are met then the player has a complete exemption from the ATP World Tour Masters 1000 player commitment.
The first Year of Service shall be the first calendar year in which a player has competed in at least twelve (12) tournaments offering ranking points.
* From 2010 only main draw singles matches played in the following events will be used in the player’s match count for purposes of this rule.
ATP World Tour
Barclays ATP World Tour Final
Challenger and Futures matches played prior to 2010 will remain on the player’s record as relates to this commitment reduction rule.
B. A player who is eligible for a reduction of his ATP World Tour Masters 1000 commitment tournaments must be in good standing with the ATP in order to receive a reduction of required ATP World Tour Masters 1000 commitment tournaments
This means Muger Federer can miss as many Masters as he likes, whereas Nadal can only miss one.